Umbridge, dementors, spy in Order?

[linocow2000]

No: HPFGUIDX 72414

Hello everyone. This is my first post so hope it's OK!
Have searched in the archive, but can't find any reference to the
fact that it  seems a bit of an amazing coincidence that the dementors turn up 
near Privet Drive just as Mundungus Fletcher hears about some dodgy 
cauldrons.

Since Umbridge admits to sending the dementors, and since things aren't
usually a coincidence in the HP universe, this suggests that she knew:
a)  HP was being watched;
b)  when Dung was on duty (unless she had ways lined up to distract
any possible member of the order - but it does seem that Dung isn't known
for his reliability).
So unless Umbridge is bugging the Order's HQ, presumably impossible
(from outside), how has she got this information? Is there a spy reporting
to Fudge and/or LV?
Have so far failed to come up with a likely candidate - can't see
Snape doing favours for Umbridge (don't think he is a spy anyway - too 
obvious),
really don't think Lupin is evil (and Umbridge hates part humans). Suppose
Tonks is a possibility, but that doesn't feel right to me. Maybe it is some
obscure, as yet silent member of the Order, but that would be a bit of a let 
down.
Any ideas out there?
Caroline