Well, once again, don't trust the wife when she says things...
So I scanned for codes, none. Then I took the car for a drive. Discovered that if you accelerate from a stop going uphill the Slip light comes on and stays on, BUT ALSO the Brake light comes on for a second or two. She didn't mention that one.
Had a look at the brake fluid reservoir again, and the fluid was lowish, not really close to the Min mark but it was difficult to see where it was because of where the seam between the top and bottom of the reservoir was. Topped it off, took another drive and attempted to replicate the problem, nada.
Calling it fixed.
Thanks for the input!