on the desktop and possibly elsewhere
…
see if there is a shortcut for Notepad++ on your desktop

a second location that Windows checks is a shortcut pinned to your task bar

Also, if @ddalthorp has AutoHotKey or similar “hotkey utility”, there might be a “launch Notepad++” action for the keycombo defined in that utility, as well.

Windows has a built-in hot-key thing that works for shortcuts that are on the desktop and possibly elsewhere. Right click on a shortcut icon on the desktop and select Properties. It should default to showing the Shortcut tab of the Properties dialog box. If not, switch to the Shortcut tab. About mid-way down is a field for Shortcut key. Select that field and do Ctrl+Alt+7 or whatever you desire. Click [Ok] or [Apply] and now Ctrl+Alt+7 will start the shortcut.

So, I would first first check to see if there is a shortcut for Notepad++ on your desktop and see if that shortcut’s Shortcut key field has Ctrl+Alt+5. If so, now you know why Ctrl+Alt+5 starts Notepad++ and can delete or change the shortcut key.