I did a test, created a one file swarm, downloaded it to another node, then deleted the file in the file system.
The sending node doesnt indicate a problem, even if I open the swarm I get what seems to be a correct filesize, the only way I found to see that it is broken, is if another node tries to download it, then the size is indicated as zero.
Even after this, the sending node still happily reports original filesize, and this after a restart
If you go into Classic ui, you get an indication that the swarm is broken, but nothin in the web ui. Whats worse, if you run on a mac, you cant see the classic ui, so then there is no indication whatsoever that you have bad swarms
Perhaps a tool to walk the swarms is needed