I too have a CDL and always use them regardless of the conditions or move. I think some people think that the turn signal is strictly for use when at a stop sign or left turns into oncoming traffic. Maybe it's a rental and they just turned on the wipers
I still use hand signals too and use them quite often when someone does not use theirs. It's usually the one that indicates they should return to the middle lane!
No turn signal is a peeve of mine and drives me nuts.
I still go to the back of my trailer before every departure to check they work.
Now merging...that's a whole other issue...