If you think that whatever is left onboard has been through security screening then the chances of it being a threat are small. However, the fact that this sort of thing happens all too often demonstrates a clear weakness that can be exploited by those that wish us harm.
Much depends on the quality and efficacy of the security screening. Having witnessed screeners texting while supposedly watching the scanner screens and other abuses, I would err on the side of caution; there is a danger in becoming blasé and always treating these things as just lost items.