Sorry, should have been more specific.
The rear wheel had not been disturbed. The front had been removed and reinstalled (front pads were pushed back some to facilitate install). I noted the venting while running the engine and applying brakes (hard) with the front lever while compressing the fork to align the front end. The fork legs had been removed for new seals.
Given this added info, what you saw is somewhat normal - although I am not on-site to confirm all work done and all symptoms.
BY DESIGN ALL BMW "K1200" and Boxers R1150/R1200 between 2002-2006 in USA (or 2001-2006 for other countries) use a similar ABS modulator design that is servo assisted:
1) the Front "Control circuit" of brake fluid is between handlebar Master and ABS modulator - there is NO venting but expansion is allowed inside by classic rubber boot inside Master cylinder at handlebar.
For the Rear "Control-circuit" , the master-cylinder cannot have a rubber boot, so there is venting/expansion from the most external section of the white reservoir showed in your picture. This is the reason why there is a small black hose between reservoir out-most section and Rear brake master.
2) the "Wheel circuits" of brake fluid are between the ABS modulator and each wheel calipers - venting and brake pads wear compensation is made by venting 2 reservoirs (1 for rear circuit and 1 for front circuit).
On Most Boxers models, both small reservoir (front and rear) are embedded into the top of the ABS modulator - on these models the ABS modulator is mounted Horizontally in frame. On K1200RS/GT and also on some K1200LT (2002-2004) the ABS modulator was mounted vertically instead - in such position they could NOT use the top reservoirs so they used a separate 2 sections white / opaque reservoir on right side (below seat).
As the brake pads wear, the brake fluid level goes toward MIN mark, When brake pads are new, the fluid level should be adjusted to MAX mark. Whenever the pistons of Caliper(s) are opened up (to remove caliper / wheel) this will makes the level go up momentarely in the specific (front or rear) reservoir.
If the fluid level was a bit too high and not proportional to brake pad thickness, there could be an overflow for one or both venting hoses. If your bike is 100% stock, these 2 small black venting hoses go up a bit from the white reservoir (tied to sub-frame under seat), then they curve down together (passing near battery box) to end below rear swing-arm to vent toward the ground (in case of overflow).