This report is a feature request for more clear and logical (or explained better) UFO notifications.
Thus, it's quite a bug report as well, because when you don't understand things, you start believing THEY bug.
This is perhaps obsolete as far as v2.6 is concerned, however.

==== 1°) UFO not notified: ====

I already read about, and experienced Supply ships not notified as being lost when they land on an alien base or alien base building site, and thus not notified as being detected again a few days later, when they take off.
I don't know the team's answer to this behavior, and in case it's not known, there are two other cases at least when UFO is not (correctly) notified:

===== Not notified appearance: =====
On the screenshot, you may see UFO#75, a not yet researched Gunboat. It appeared from nowhere, targeting a transport. Thankfully, it was caught by a SAM network and shot down.
Usually, when an armed UFO target a PHALANX craft, you get the notification:
MSGID#2708 "A UFO is flying toward %s"
In this case, I got:
MSGID#2709 "%s is flying toward %s"

===== Not notified disappearance: =====
Secondly, I've built strong doubts about MSGID #2711 "Our radar has lost the tracking on %s", because I get several UFO disappearances in my log, without any notification. Then, what happens when an UFO finishes its mission on Earth and flies into orbit. Shouldn't we be notified that the radar lost it?


'''Suggestion:'''
# Check in which conditions both strings are used. Either #2709 shouldn't be used at all (indeed, it's never been used, save in this #75 case), or #2709 should be used when UFO's type is known (having been researched) and #2708 should be used UFO's type is unknown, the same as when %s (UFO's name) is either #60-#80 when researched, or #2294 when unknown.
# Add a detection notification #2710 "Our radar detected %s near %s" as it's always the case.
# Check if an UFO (Harvester, Fighter, Scout, ...) disappearing from the geoscape is always notified as being lost, whatever happened (flying out of radar range, flying back into orbit, landing unseen, ... cloaking!)

====2°) Hierarchy of notifications:====

Second grievance is that the logs (the geoscape overlay) is sometimes hard to review, because the icons used to notify alien activities are quite confusing (imo):

Blue mark is for:
* UFO being lost: #2711 "Our radar has lost the tracking on %s"
* UFO being lost to sea: #2321 "UFO interception successful--UFO lost to sea"
* Reco, Harvest and Corruption alien activities (landings): #2565 (?) "Alien activity in %s"
* Alien base detected (generating a ground mission): #2565

Red gray head is for:
* UFO being detected: #2710 "Our radar detected %s near %s"
* UFO being shot down (generating a ground mission): #2565
* Terror alien activities (landings): #2565
* PHALANX base attacked (not every time)
* PHALANX craft shot down ? #2324 "You've lost the battle"

About the Terror missions, I'm not really sure if the icon depends on the UFO type, or on the fact this is a mission without UFO.
(That is, I'm not sure the icon is always the red head one).

'''Conclusion:'''
As you may figure, it's not so simple to review the log and to assert what happened without taking notice to the dates and the way events are chained.
In peculiar, it's hard to understand why a Terror mission is notified the same way as a crashed fighter (or even a scout), whereas Corruption and Harvest missions are notified like the end of a threat (UFO lost to sea or UFO's signal lost).

'''Suggestions:'''
# Except if the hidden nation approval impact is different by a large margin, notify any alien ground mission like a threat (alien activity on Earth: landings, crashes, bases): red gray head. The UFO detection fit to this category as well (being an impending threat).
# Notify any secondary information in blue: same as currently, less the alien activity.
# Possibly, add a "UFO interception successful"(--UFO crashed on the ground) notification (#2320 "Interception" not used?), (the same way #2321 "UFO interception successful--UFO lost to sea" is used) to help discriminating between a crashed UFO and a landed UFO (Alien activity) in the log. Thus, a crashed UFO would generate two lines "UFO interception successful" and "Alien activity in %s", whereas a landed UFO would generate only the latter.


'''Other suggestions:'''
# When an interceptor is shot down, rather than a generic "You've lost the battle", #2324 could be more precise: "PHALANX craft was lost" or "You've lost a craft" (e.g.)
# When an UFO is lost, the name of the radar station could be added, the same way it's added when an UFO is detected. Thus, #2711 would become "Our radar has lost the tracking on %s near %s" (cf. #2710, see above)
# When an UFO's type is known, the alien activity could be written in clear: create another MSGID "Alien activity: %s in %s", to replace #2565, the first parameter being any new MSGID from "reco", "harvest", "corruption", "terror", etc. E.g. "Alien activity: harvest in United America". Granted, while the activity is still not treated, we have the geoscape icons to inform us, but then, the log should really be more clear about that.