Greg wrote:

When the roto-rooter guy came out, the first thing he said was
"did the water empty out of the toilet bowl, because that's usually the
first sign".

This is because water backs up in the pipe far enough where it interrupts the
venting process. Then siphon action when using other drains will suck water out
of the traps, the toilet being the most noticable one.
The flip side of that is you may also see bubbles or gurgling in the toilet as
the pipe fills beyond the venting level

Greg is 100% right.

In the case of the OP, it didn't sound like any backup problem though.

Slow draining of the trap water is usually due either to a foreign
object (cloth or string for example) in the bowl trap or an internal
crack in the trap. In the string case, the string acts as a
wick to siphon water out.
(I like the dog explanation too!)