Lol, the same thing happened to me when I brought my baby girl, Lilah, home on her first day :)
She's unusually tiny for a ferret and as a baby she looked more like a big mouse. Anyway, I have a ferret cage which is around five levels I think and I'd already had my boy ferret for a week at that stage (he's quite big) and hadn't had any problems so I popped Lilah in the cage, watched her for a bit, then turned out the lights and left them to settle in, closing all the doors behind me.
While they were babies I kept them and their cage inside as I didn't want them out in the elements - they both live outside now.
Anyway i came back after maybe 15 minutes to check on them and Lilah was gone! Chubby Heartspark was sound asleep but Lilah was nowhere to be seen! After a few minutes of looking my sisters and I found tiny Lilah scuttling around under the bed and after checking she was OK, we put her back in the cage.
This time we stayed and watched and to our suprise we saw her lie on her side and wriggle through the bars! These bars were only 1.5cms apart at the most, she's like a contortionist ferret! She ended up spending her first week with us in a bird cage until she'd grown enough not to escape.
She can still get through anything more than 3.5cms or so...
