Owls to hidden places/people

[Deb]

No: HPFGUIDX 111428

Here's something else I'm wondering about: Owls are obviously VERY 
smart, since they can find people whose whereabouts are unknown to 
the people writing to them. Harry sends letters off to Sirius via 
Hedwig, but he also sends him mail with the school owls in GOF when 
he's hiding. So, owls obviously have some special instinct in finding 
people in their postal duties.

Taking that a step further, two questions:

1. If an owl can find a man on the run from dementors and part of a 
huge manhunt (CoS), why did no-one in the ministry think to send 
a "Dear Sirius, hope you are well" letter and just track the owl??

2. But also . . . Hogwarts is unplottable, yet it receives mail. 
Fine. But what about places and people who are hidden by a Secret 
Keeper? Can people send mail to the OofP headquarters without knowing 
the location? Can they send mail to anybody LIVING at HQ if they 
don't know. Or if they do know the secret--like Harry when he writes 
Sirius at the beginning of OotP--does that mean the owl is let into 
the secret, too? And, say, the Potters 15 years ago . . . in the 
brief time between casting the spell to set up Wormtail as Secret 
Keeper and the time Voldemort showed up at their door . . . could 
they receive mail? How? Did their Daily Prophet arrive??

Deb in NJ