Dog Pees Inside During Thunder Storm?

I have a 13 year house trained Dalmatian/Lab mix. I have had him since he was 4 months old and have never had an issue with him using the bathroom in the house.

Last week we had a thunderstorm in the middle of the night. At the first big clap of thunder he went to the foot of the bed and peed on the carpet. He did this again earlier this week. The scenario was exactly the same.

Yesterday I came home from work and he had peed on the floor in the hallway. There was a thunderstorm while we were gone. Today we went to the store and it began raining while we were gone. I'm not certain it thundered, but he peed again in the hallway.

I am not sure why this is happening? Is he scared? I know he is old. Maybe he is losing control of his bladder?

Any insight would be appreciated.

Thanks!