Yes, it is certainly true. Spybot S & D and Adaware, run regularly, ought to pick up any malware on the system, though just like AV, it can sometimes take a day or so for the definitions to be updated, during which period the malware will remain undetected.

The entire point of any type of spyware is to send/receive information to and from yout PC. So no matter how carefully the coding is crafted, the malware will always be detectable by using a packet sniffer. The only undetectable spyware would be one that didn't exchange any information, which would render it harmless anyway.