Yes, everyone sees one overhead on occasion, but this is *not* 28 Days Later...it is not a sign that you've been quarantined and civilzation is still trucking along just over the horizon. Rather, it is a sign that someone desperate and lucky has managed to fuel up a plane in an attempt to travel a long distance or move a lot of goods or both. Various reports suggest that over 50% of these flights end in a spectacularly failed landing attempt, if not an outright crash. I've seen a vid someone took of Collapse hitting a 757 *in flight*.
Most flying things don't trigger Investigate - so if you have an ultra-light, or a tethered hot-air balloon (if it's un-tethered, you're taking a big gamble on whether or not the spot you land in is secure or not), these are amazing for scouting or observation - but they *will* attract survivors (who may or may not be the friendly type).
Even better than the above are self-contained vid recorders - attached to a kite or an RC plane you can get a lot of recon with minimal risk - if you're lucky enough to find the gear.