Why? Because...
 

1. The information are no longer up-to-date (peers might have gone offline or Pushroutes within horizon are no more valid).
2. To show newbies that the search has definitely runned out (see above).
3. It's time to start a new search!

Results of my investigation
 

+ 5 min 36 (minogue, v) 5 (tom jerry, v) 1 (diary dreams, a) 15 (pizza, d)
+15 min 90 (minogue, v) 6 (tom jerry, v) 8 (diary dreams, a) 18 (pizza, d)
+30 min 128 (minogue, v) 11 (tom jerry, v) 8 (diary dreams, a) 31 (pizza, d)
+60 min 208 (minogue, v) 19 (tom jerry, v) 12 (diary dreams, a) 52 (pizza, d)

Description: Elapsed_time ... hits (keywords, media type) .. more hits
(Choosen media types: v=video, a=audio, d=Document)

Closing those search windows, and immediately performing a new search gives me a completly new result. The hit count is then similar to the first line above.

So my interpretation of this behaviour is: Xolox does NOT run periodical researches, but run the search queries on every new connected gnutella host. While hosts drops and get reconnected time by time it looks like a research... but I have never seen the hits going down again (right?). So the seen behaviour looks more like an accumulation of all old hits.
That might be the reason why Xolox search window should be closed after a long period of time (e.g. 1 hour). You would see old search hits, which Horizon, Peers or Pushroutes would not longer exist.

Automatic researches are performed for active downloads only IMHO. Can someone confirm this?

Greets, Moak

Even if the search window only accumulates all old hits, I still find it very usefull for searching for very rare files - they at least show up in the search windows. I then tell them to download and xolox can do automatic searching for the file even if the user that had it is offline by then (another user might have it or it will stard downloading when the original user reconnects).

I don't have time to search for files I want every 15 minutes so leaving the search window(s) open overnight very often results in finding even very rare files. It's one of the best features of Xolox and I would hate to see it go.