Thanks! TBH I can't think of any other way air could get there except from the returns, as you suggested. Think I will do the two-part epoxy coupled with a pool leak sealer (eg https://www.amazon.com/Marlig-Industries-Pool-Leak-FBA_FAL_32oz_C14-79-CA/dp/B004BN89GE) to find any potential gaps in the return.