JIRA Quick Search and the issue key prefix
Understanding how JIRA Quick Search works
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. If you have recently done actions in a project like viewing an issue, that project becomes your 'selected project'. JIRA tries to 'guess' which issue you are looking for, given the 'selected project'.
But if you have just logged in to JIRA, and not yet gone to an issue or a project, you will need to type the complete issue key (including the prefix).