That's what I suspect. 2012 TDV8 Vogue (L322)

I have a similar problem. If waiting at traffic lights, that I know will not change for a while, I tend to switch the engine off. If I have my foot on the brake pedal, it presses back with the result that the vehicle starts to creep if on a slope. This has only manifested itself in the last few weeks. Braking system otherwise OK. Bit alarming when it first occurred but ready for it now. Previous cars:
S-Max 2007-2013 (only diesel I've had, good car but expensive when diesely bits go wrong, so what's the point?)
Galaxy 2001-2007
Mondeo Estate 1997-2001
Sierra Estate 1993-1997
Uno Turbo 1987 -1993
Fiesta 1984 - 1987
Fiat 127 1982 - 1984

Mine does it, Brake pedal won't depress far after being left for a few days. Never had a problem starting the engine though and of course when it starts eventing returns to normal. Geoff

Mine started to have this about 6-8 months ago.
At the end of 2023 I replaced the rear rotors and pads and bled all 4 corners, the rear pads were pretty rusted, so was the contact area of the rear pistons.
That made me think the expanding rust pushed the brake fluid back during longer parking times creating the pressure.
I mean, iron/steel doesn't stop rusting, rust is not airtight.
With everything rust free and the contact areas coated in anti seize paste I was hoping the problem would be gone.
For a short while it seemed to be not as bad but it returned to how it was before.

What I also noticed is that when I keep full pressure on the brake pedal after starting the engine I can press the pedal, albeit slowly, ALL the way down.
Is that bad?
Once I let the brake pedal come back up and pump twice it's all normal.
Braking is normal when driving and while keeping the car stopped at traffic lights. 2012 4.4TDV8
2000 Honda S2000