My point exactly, the makers of that movie had no idea what they were putting to screen. First of, the Pred's have advanded tech and are supposed to be fighting Aliens for centuries. With all the varied scanners in their helmets I would suppose they have a specified Alien scanner as well. Shit, they should have it around their ships like smoke-detectors.
No f#cking way Pred´s should get surpised by a Alien sneakin aboard.
Second, even us humans have brought many far more physical powerfull animals on the eve of extermination by use of technology, and we are to believe Pred´s see any real danger to their species in Aliens, come on...