When you proxy via ZAP you will often see that some of the Ids in the History tab are ‘missing’, e.g. it will jump from 1 to 4 etc.
The missing Ids do not refer to ‘hidden’ requests that ZAP is making.
Instead those requests (which are not sent at all) are generated by ZAP for “internal” use only. They are used to show a “GET” request when “directory” nodes of the “Sites” tab, not yet (manually) accessed, are selected.
So if you access
http://www.example.com/images/123/456.jpg then ZAP will
fill out any ‘missing’ nodes in the tree, e.g. the ‘images’ and ‘123’ nodes:
[+] http://www.example.com [+] images [+] 123 [ ] 456.jpg
The history tab may well show something like:
1 GET http://www.example.com/ 2 GET http://www.example.com/images/123/456.jpg 5 GET http://www.example.com/xyz
In this case the Ids 3 and 4 are used internally for the ‘images’ and ‘123’ nodes added to the Sites tree.