Just came across this today.
I was watching my death replay just as our base was being sieged, and when I exited out of it, everyone else had disconnected but I was still counting down to respawn.
I spawned into the map and could move around as normal, but the hud was gone and there was no sound.
I didn't stick around for long as pausing gave the option to quit "spectating", but I am guessing I could have stayed in that match indefinitely.
I was watching my death replay just as our base was being sieged, and when I exited out of it, everyone else had disconnected but I was still counting down to respawn.
I spawned into the map and could move around as normal, but the hud was gone and there was no sound.
I didn't stick around for long as pausing gave the option to quit "spectating", but I am guessing I could have stayed in that match indefinitely.