"I would suspect a very minor leak at the bleeder nipple or a banjo fitting. Can't be too bad or you would see brake fluid dripping somewhere."
Don't believe this would introduce air into the system except for possibly the linked front caliper.
For the front brakes operated by the hand lever the master cylinder reservoir is at the highest elevation in the system. If there's a small leak somewhere in this system air wouldn't come in, fluid would leak out via gravity from the master cylinder reservoir. Once the reservoir was depleted then air could enter.
The linked system is a bit different. Some portions of the hydraulic line feeding the front right caliper are at a higher elevation than the rear master cylinder reservoir. If you had a leak in the front linked caliper air could possibly be introduced since fluid wouldn't necessarily flow from the rear MC reservoir.