> This article [1] says tcpdump will output the packet counts, so it might also flush buffers, I'll try to check and report a little later today.

I checked, tcpdump doesn't seem to flush stdout on siginfo, and hitting ctrl+T doesn't deliver it a siginfo in the tcpdump | grep case anyway. Killing tcpdump with sigint does work: tcpdump's output is flushed and it closes, and then the grep finishes too, but there's not a button to hit for that.