.
Here is what I'd ask:
1, What is the business purpose of this app (if you do not already know);
1.5, how to use it
2, Archticture diagram (even if hand drawing is okay);
3, design diagram if possible;
4, how to get access to source code (if exists);
5, most frequent support issues/bugs;
etc, etc...
然后向老板汇报 what you have leared, and CC him/her.
职场 Rule #1：nobody is indispensable ...
Rule #2: refer to Rule #1
