Probably all have encountered on occasion.. not a typical leak that reveals itself easily, but the type experienced after a week or two, tire pressure is 8 lbs less than when last checked...?

The smaller leak can be tough to spot, even with tubs and water, they can escape detection. Memory of vintage gas stations are relevant, formerly known as mechanics with soap and bucket combed over the tire. But some things can be overlooked if not careful :

You must be outside, with good light. There must be a chair and patience. You must have something larger than a cup, a good size brush is best, not kitchen size. After beads and stem checked, the task is to gaze upon bubbles - likely one will start to expand 'slowly' in size. These are the nagging leaks, which require combing the circumference slowly to spot them.

Repairing the leak :

A manual changer often takes significant effort to break, dismount beads, but satisfying when finished. After dismount, you don't need professional plugs, so long as you roughen the inside of tire with rotational stone grinder for drills. You do need a quality adhesive, which weatherstrip seems to work well, so long as your soft rubber material follows the contour of a tire. A halogen work light, some heat will help bond things before remounting the tire.

Finished a repair just now, fresh insights may prove useful..