The vote scanners aren't connected to any networks, unless you're speaking colloquially rather than the network security meaning. At that point - yes, there's a way to notice. People who open the envelopes tally the votes while they're doing it. This is prone to human error of course, but it helps to detect wildly inaccurate swings in the vote totals. If you pulled out the memory sticks / security tokens necessary to upload the data to their final destination, you'd be able to notice that someone accessed the machine/pulled the sticks between pickup and dropoff.
The woman in Georgia who showed herself on the computer her adjudicating other peoples' votes for particular ballots wasn't herself sure of whether the software actually would overwrite the scanned-in votes, and the footage from Delaware county PA shows that rejected ballots are filled out on completely new ballots by two people of different party affiliations with a security guard watching - one reading, one filling it in.
With all that in mind, for someone to mess with the vote tallies either on a level that would influence elections, you would need complete cooperation among virtually everyone in an entire area to a degree that boggles the mind. And this has to be somehow more plausible than "the guy who barely won against
Hillary Clinton by 80k votes failed to make significant inroads outside of his base demographic."
That and it removes the guarantee of private voting,
Using PA as an example, I know here
I can look up how anyone I know voted. The situation you're describing sounds like it's either someone without a spine or in an abusive situation, and an abuser will check the records through this manner. It does lower the barrier of entry to some degree, sure, but for every person whose vote gets beaten one way, you could argue there's someone else who will cast a vote that might otherwise not have bothered.
Forged signatures/someone fraudulently mailing someone else's votes is technically detectible
It's imperfect, prone to human error, and a fairly faulty system. Depending on what state you're in, though, you already didn't need to provide any form of ID to vote (some do require proof of it). The Cobb audit found that there were a whopping two signatures that deviated largely from the totals, due to a woman signing for her husband and some idiot signing in the wrong spot. There is no doubt that some degree of fraud could have happened with the issuance of these ballots, but the burden of proof lies not on finding a single instance, but on a repeating pattern of peoples' ballots being mailed to improper addresses, actively used, and not flagged by the signature comparisons - to a degree large and wide enough to affect elections.