My first guess is a loose valve core.

Next would be a breach by where the valve stem meets the rest of the inner tube. I've had those and even when submerged in water the bubbles didn't appear until pushed the stem off-center.

Next guess would be a thorn that has worked its way through the tire and is ever so lightly impinging on the innertube. When sitting still its a slow leak, when riding it leaks much more. Take off the tire and feel around the inside of it for a small, sharp point.

These have all happened to me over the years.