I recently had an incident - hit a high tension power cable that I didn't see coming in in my goggles. I lost OSD and RC link immediately and the drone fell into a rough terrain. I recovered it however pretty easily because the battery didn't detach and the buzzer (not autonomous) was beeping constantly.

So this led me to a question - how is it possible that the FC apparently went into a non-operational mode (no OSD and RC link) but was still controlling the buzzer? Would it be possible that after a crash even an autonomous buzzer wouldn't help because the FC stops working, the battery don't disconnect, and you cannot control the buzzer from the transmitter and the autonomous buzzer is not engaged because it's still under power?

After unplugging and plugging back the battery my drone was just fine, except for a IR filter detached from the camera lens, that I had to disassemble and glue it back.

It sounds like your quad failsafed. When it does that, if you have the RX_LOST beeper option switched on then the beeper will start to beep continuously until you unplug the LiPo. This is so you can find your quad after a failsafe, assuming that the LiPo is still connected in order to power the FC and buzzer.

An autonomous (self-powered) buzzer works differently in that if you completely disconnect power from it (i.e. your LiPo gets ejected and disconnects itself from the quad), it will start beeping using the power from it's own mini LiPo. It doesn't require connection to the FC to do that.

Don't think this was a failsafe - I completely lost video, OSD and transmitter signal.

I get this all the time on a betafpv FC. Any good hit just makes it reset like I removed the Lipo and reconnected. Nothings loose, Lipo makes good tight contact through the xt30. Id always assumed it power cycled but I've never really paid attention if it failsafed intermittently or changed osd settings if I'd tweaked them from the start up default. I suppose it's possible.

This seems to happen to all my toothpicks but not so much to larger 5" quads. These are usually softer mounted in my builds. My toothpicks have the canopy mounted right on the FC stack so impacts will send vibrations down the stack bolts. I'd always thought this was normal behaviour I attributed to a large shock on some FC component.

How are your FC mounted?

In my case it didn't look like power cycle, after the hit the FC was like completely dead except for the beeper. The FC is soft mounted.