So I had a backup in my line from washer and utility sink. when the washer drained, the sink would fill slightly, then empty immediately. Had plumber out wednesday. He said it was septic. Emptied septic, still backed up (somehow it was more than before) so had plumber back out friday. They took apart p trap, realized they had to install a cleanout further down to get the rod through, then put it back together, did the fix through cleanout, and left. I noticed a leak from utility sink so I called them back after confirming location with a paper towel. It was at the bottom of the back nut. They tightened it and there wasn't anything visibly leaking, so they left. 2 hours later I noticed water underneath again (just a teeny puddle) called again and they said bx its so slow and the time of day I caught it, put a bucket under for weekend, and they came back today. They put 2 buckets of water through sink and nothing leaked so they said its not logical its leaking when water isnt going through it and not leaking visibly when it is. I pointed to the fact there was water in the bucket over the weekend but it was just a small bit and even evaporated between yesterday and today. He said he didnt know but its fine. So I figured maybe it stopped. I even tried a paper towel right after he did the water, but it was dry. So he left. But when I put a paper towel on the area after he left, it will wick away water. Its just a small bit and a while between before its wet again. Takes a while to accumulate. I emailed the company owner (bx thats the only email listed and I wanted to send visual proof of what im experiencing, not to get him in trouble or anything). Wondering what this could be. I even asked if it could be a washer needs replaced after he opened the nut for the first time in at least 5 years (haven't touched it since we purchased our home) and he said theres no washer, so that cannot be it. He refused to touch it bx he couldnt visualize a leak so he said nothing he needs to do. I feel like an AH at this point. But its a leak even if tiny.