What does "mechanic tinkered with the brakes a few weeks ago" mean?  What
did the mechanic do?  Why?  Did you take the car to the mechanic because you
were having brake problems?

Your problem has nothing to do with the engine being cold.  It's most likely
nothing more complicated than the pads having set on the rotors overnight.
That is very common in a large number of cars.  Cheap pads are more prone to
this than a decent set of ceramic pads.  If the mechanic replaced your pads
as part of the "tinkering", then it would be worth finding out what kind of
pads were used to replace yours, and whether the mechanic installed the
shims behind the new pads when he installed them.

The reason they stop making noise after you apply them is that any slight
rust that begins overnight, or any slight set that the pads take to the
rotor overnight, is eliminated by applying the brakes.  The pads are pushed
into the rotors, aligning them flatly against the rotors, and preventing
them from oscillating, and any buildup on the rotors is scraped off,
allowing for a complete contact area between the rotor and pad - again
resulting in eliminating any oscillations.