the rules at the center where I used to teach water aerobics are as follows: if anybody in the facility (not just the lifeguard) hears thunder or see lightning, the indoor pool will be closed. It will reopen 30 minutes after the last evidence of a thunderstorm.
I know that they are checking the direction of storms with the Doppler radar to advise members who are calling in to inquire. But unless there is audible or visible evidence, the pool remains open.
The rules for many pools are such that if lighting is present they’ll need to be remained closed until the storm has passed or it has been downgraded. This was in effect even when I swam for the University of Alabama 25 years ago. Their aquatic center had to close down (it’s indoor pool) when there was a lighting warning. During all other hours of operation the lifeguards were responsible for closing down the pool when thunder was present.