Coincidentally, I just spotted this article on the BBC website:
BBC Nature - Virgin births discovered in wild snakes

It refers to Warren Booths work, but if you read through it all there's a mention of a checkered garter snake as well. It's not quite on topic (ie stored sperm). It's well written and easy to read though.
If the frequency of facilitative parthenogenisis is higher than we thought it could be another explanation for litters born when there have been years without male contact. Sperm storage could be a shorter term mechanism, with FP being more likely for litters born after a longer period without male contact.