If the user execute less then hit the "F" (follow file), the file being monitored is polled regularly to get updates.
If the user first added a trap to ignore SIGHUP before executing less, then closed the terminal abruptly, this leads to still having less monitor the file "in the background", despite the user not having a session anymore, instead of quitting.

Reproducer

1. Open a terminal
2. Set a trap to ignore SIGHUP

   $ trap "" HUP

3. Execute less on a file

   $ less some_file

4. Hit "F" to follow
5. Close abruptly the terminal

Expected result

less exits on its own.

Actual result

less remains in the background:

# ps -eaf | grep less | grep -v grep
root        6427    6372  0 13:05 ?        00:00:00 less anaconda-ks.cfg

# strace -fttTvyy -s 128 -p 6427
strace: Process 6427 attached
13:06:00.084445 restart_syscall(<... resuming interrupted nanosleep ...>) = 0 <0.178072>
13:06:00.262753 read(4</root/anaconda-ks.cfg>, "", 6614) = 0 <0.000090>
13:06:00.263119 nanosleep({tv_sec=1, tv_nsec=0}, 0x7ffddb156c20) = 0 <1.000318>
13:06:01.263715 read(4</root/anaconda-ks.cfg>, "", 6614) = 0 <0.000070>
13:06:01.264066 nanosleep({tv_sec=1, tv_nsec=0}, 0x7ffddb156c20) = 0 <1.000232>
...

The reason for behavior is SIGHUP is ignored, IMHO less code should restore the default handler for SIGHUP.