Personally i love ratties and I AM NOT SURPRISED it got through your defences:
"The Power of Rat Teeth: The jaws of a rat can exert pressures of 24,000 lbs. Rats can readily chew through plastic water pipes, irrigation systems and garbage cans, wood boxes, dry wall, and even cinder block. Compare the hardness of rat teeth with that of common metals. On the international "hardness index" rat teeth = 5.5, while iron = 4.0, copper = 2.5-3.0, aluminum = 2.0 , and lead = 1.5. It was once believed that rats needed to gnaw to maintain their ever-growing incisors (front teeth) at the proper length, but this is not true. Rats fed on soft foods and kept in an environment without gnawing edges do just fine because the teeth wear on each other. Rat teeth grow 5 inches a year, however, and rats with mis-aligned jaws have been know to pierce their own brain cases with their own rapidly growing teeth."
Is it any wonder the rat got into your compost bin??? the pressure poundage is wrong in this quote, i think its a typo. i believe it is actually 24 lb (i know that if they put their mind to it they can take off a human finger very quickly!) but the rest of the quote is accurate. rats incisors are enamel at the front and dentine at the back, thus the back wears quicker producing a chisel tip, which is extremely hard, which is why they can chew through anything, including metal and concrete. if they were the size of a shark they can bite harder.
if you're rat is just visiting tho, and not nesting, i wouldn't stress too much about it.
HOWEVER, they are not really a good idea to have it nesting in your garden, ... you do the math:
one female = up to 18 babies every 5 to 6 weeks.
each baby = sexual maturity by 5 weeks. each can have up to 18 babies
each "grandchild" = sexual maturity by 5 weeks and so on.
they live together so it's not gonna take long before you are overrun. so make sure it isn't nesting and keep stirring the compost.
you can get a humane trap. your rat will have a set route to your compost bin, you just need to watch which way he/she is coming and put the trap on route with some nice smelly food inside (ie. chicken). check the trap twice a day but not more than that. my own ratties are active early in the morning and late at night (between 10pm and 2am) so check the trap outside of these times. if you catch it, relocate it 2 miles away.
the good news is, if you have a rat you don't have mice, and rats are much cleaner. it's true they can carry weils disease (pronounced Viles) but that's usually sewer rats and it's usually sewer workers that pick it up. It is very nasty tho, and can kill, so as said, as a precaution make sure you wash hands after dealing with compost.
ps. tell your misses not to stress. rats are extremely intelligent but shy, and although they will defend themselves if cornered, they won't go for her jugular! and they will see her coming before she sees them and run away.