[2011.2017.odyssey]

overfilled or pumping the pedal too hard after the service?

clean up the area with some brake cleaner and observe.

at first glance it seems like the master rear seal.
certainly something you want to keep an eye on.

 

[2011.2017.odyssey]

All good info above... if someone topped up the reservoir with brake fluid because it looked low.
BTW never do this.
when you compressed the calipers for the new pads, this could cause an overflow situation.

 

[2011.2017.odyssey]

if you look below the master reservoir, you will see some fluid in the bolt hole below.

if you touch the bottom of the reservoir it is likely wet, some fluid overflowed went into the hole below and trickled along the bottom to the brake booster.

clean things up and see if anything returns.

Edit: looked a little closer that reflection in the hole looks more like a scratch in the aluminum.