This may be a variation on the problem reported by 'TheRawGod' against build 1522 ('VC6 Incorrect Suggestions / Missing Members').
I have two unrelated projects that both declare a class named CSysParms to access parameter settings for the application. Each project has different member functions based on the names of the associated settings. In build 1526, the suggestion box that pops up in project A includes the member functions for CSysParms from project B as well.
If you open a file from another project, the file and everything it #includes get parsed and the new symbols are accessible from your current project. However, VA X "should" forget about these files and symbols the next time you load your project. There was a bug in 1527 and earlier that prevented the symbols from being forgotten untill our symbol db was rebuilt.
Can you give build 1528 a try and letting us know if it fixes the problem? (No need to reset your IDEs during install.)