When I was playing in an Irish folk band, one thing we did each March was visit elementary schools and play music and talk a bit about Ireland in an attempt to get away from the image of dancing leprechauns and green beer and "traditional Irish food" like corn beef and cabbage.
One year, we were playing for a room full of kindergartners when one of them asked "Are leprechauns real?" The teacher smiled and chuckled a bit and for some reason, the other four guys in the band looked at me and one said "This one is yours Pete."
I looked at the little girl who asked the question and said "Just because you don't see something does not mean it is not there." This made the teacher smile and nod. It also got us out of a pickle.
A few days ago, our tomcat, Pumpkin, was staring intently at something neither my lady-wife nor I could see. He was clearly watching something, and it was moving. He looked precisely as if he was stalking something. My lady-wife asked if I knew what he was watching - I had no idea.
Now, we live with three cats in the house. All of them, at different times, will watch something very intently. The fact that the humans could not see anything did not matter in the least.
Software is a bit like that. You know something is wonky and you can stare all that bit all day knowing something isn't right. And not see a blasted thing.
You know something is there. You see bits that don't seem right. No one else seems to see it. You see odd behavior and sometimes you can recreate it - but often, you repeat the same steps and ... nothing is there.
So you keep looking. You might find it. You might lose interest and move on. I find it a good idea to write myself a note on what I saw and what I thought might be factors in the behavior.
Because it is likely to come back again.