Significant drops in traffic can be caused by dropping keywords in search engines. It is hard to detect a penalty from analyzing traffic.
Are you talking about loosing backlinks? if that, it could not happen
I'm sorry to say that there is not one bit of what you said makes any sense to me - so I would appreciate your clarification...
What do you mean it is hard to detect a penalty from analyzing traffic? Its the first obvious sign.... If you have traffic stats which is x per month - which is repeated month after month noting upper/lower ends and then a massive drop with a vertical line - what does that tell you???? Its not rocket science....
For example:
Then you go onto stating that you cannot lose backlinks... This happens all of the time e.g. web site changes content removes your backlink, website expires etc... There are tons of reasons why it happens... All links which come into your website are not owned by you... In otherwords you have no control over them... You thinking it never happens tells me one thing... and that is you have no idea on what you are talking about or you are confused ... I assume its the latter with letting you off...