I was looking at my Apache access log and came across some strange requests. A single IP address will access several resources (mostly css style sheets and images), but no actual pages.

Sometimes they are requesting a resource that no longer exists on the server, or one that is still under the web root but no longer used (e.g. a resource in an old WordPress theme).

Also:

• The requests list no referrer
• I get no useful information on the IP address by looking it up
• There doesn't seem to be any pattern among the IP addresses that are making these requests (e.g. different countries)

Are these just links from a stale cache somewhere? Could it be a sign of an attack of some sort?

Here is a typical example:

GET /wp-content/themes/my-theme/images/old-image.gif HTTP/1.1" 500 809 "-" "Mozilla/4.0 (compatible;)"

This was one of about 10 similar requests, some for existing resources, some for older resources. There is no other sign of this IP address in access_log.

Note the internal server error, which is a topic for a different thread. What I'm asking here is where would isolated requests like this come from?