Because you didn’t give it back to them when they got home? 
Given that it’s not necessarily obvious to the casual observer who sees you taking their mail whether or not you’re authorized to do so, I don’t think the OP’s question is totally unreasonable—though I’m not saying he should worry about it.
By the time trouble arrives for you, the neighbours will be home to say “We gave our permission.”
An FYI. The USPS has an online hold mail form. I needed to leave unexpectedly, filled out the form about 0500hrs and mail was stopped for that day and all was delivered in a bundle on the day the form I provided said to resume delivery. In the recent past hold delivery paper forms have been, more often that not, ignored. Perhaps my experience with the online form was a one off positive.
That’s actually a more complex question than you might think. Your neighbor is ( we presume ) an adult who is aware of her physical conditions. However, you’re not diabetic till the day you are diabetic. While symptoms may emerge over weeks or months, until tested you’re not A Diabetic.
If she needed a sugar for baking and was unaware that eating sugar-laden baked goods would drive her into a diabetic coma, I would hope that you would not be responsible. Then again, you are legally liable for providing to outsiders with negative results anything in your home. Give a kid a beer? You’re Providing. Give a 6 year old a peanut butter cookie that results in anaphylaxis and death? You’re liable.
Local laws being what they are, I’m painting in broad strokes and am not a lawyer. But fair to say you are liable for your interactions with those who appear at your door step.
ETA: In many areas, the information of exactly how long you are aware and the house is empty is NOT information you’d want a heap of utter strangers to have. Me, I’d ask a neighbor. If I had any I knew. 
“And in other news, police investigating the mysterious deaths of the neighbor couple are still attempting to trace the mystery woman seen intercepting their mail.” :eek:
According to US Code it’s illegal to “steal” from a mail receptacle, but if you have the neighbor’s permission it wouldn’t be stealing.
It’s ridiculous but if you really want to cover your ass you’d need something in writing from them giving you permission, but for all practical purposes picking up a neighbor’s mail is done all the time and you won’t get in any trouble whatsoever for it.
I’m not a fan of allowing pics in the posts here, but this is one of those questions where you just want a to post a “facepalm” or “R U Serious” pic in response.
Oh, come on. It’s not that ridiculous a question. I heard all the time when I was growing up dire warnings about the illegality of “tampering” with the US Mail, including everything from putting stuff in the mailbox that wasn’t mailed (like fliers) to taking stuff out of peoples’ mailboxes, all of which actions were, it was claimed, deeply serious federal crimes. I’m not saying the warnings were accurate, but those ideas are certainly out there in the culture.
Yes it is.
