If Bitcoin is not stolen from an address for some months does it suggest that malware did not capture the key? Some months ago, using bitaddress.org (offline but using my normal operating system), I made a key pair, copied the key and address to paper and stored a small quantity of Bitcoin at the address. Later, of course, the computer was again connected to the Internet, but the money has not been stolen.

Does the fact that probably no malware captured my private key those months ago suggest that the key is safe? Is it credible that malware would capture an address with small amounts of Bitcoin, but not extract it in the hope that later more would be paid into the same address?
It is a fallacy of the alternative disjunct to assume that your private key is in fact private, just because a potential other party has not used it: The two events "somebody has gained access to my private key" and "nobody has used my private key" are not disjunct.

If you are uncertain whether your private key has been compromised, you should move your bitcoins to a securely created new address.