From memory, I think the routine is to stop at the desk inside the lounge to ask for a towel, as they are kept in the cupboards behind the desk rather than inside the shower cubicles.

In part, this is because the showers aren't cleaned between every use - so don't be surprised if the shower is still wet etc. Nevertheless, I've not yet encountered a shower that's been in an unacceptable condition, and they do the job very nicely.