QuickSearch guesses the issue key prefix (sometimes)
The Quick Search box (at the top-right of your JIRA screen) can sometimes find issues when you type just the number (e.g. '53'). Other times, you need to type the prefix too (e.g. 'JRA-53').
This is due to the concept of a 'selected project' - a bit of JIRA magic if you like. Basically, if you have recently done something in a project, that project becomes your 'selected project'. JIRA tries to 'guess' which issue you are looking for, given the 'selected project'.
But if you've just logged into JIRA, and not yet gone to an issue or a project, you will need to type the complete issue key (including the prefix).