lateral thinking - Mysterious message from the NoSlaves™ Calculator Co

So my son is starting to show an interest in mathematics, and being the awesome dad that I am, I decided to buy him a basic calculator. Being the cheapskate that I am, I ordered the cheapest one I could find, which happened to be from NoSlaves™ Calculator Co. ^{(When only the cheapest calculator will do, make sure you buy from NoSlaves™, where we lovingly craft our calculators to almost adequate levels of quality, and totally don't use any slave labour... at all.)}

Unfortunately (and surprisingly), when it arrived, there seemed to be something wrong with it... I plugged in a few quick test calculations:

1 + 1 = 1
4 + 3 = 9
8 - 3 + 5 = 6
24 + 51 = 84
50 - 74 + 39 = 98

Now, I like to complain as much as the next guy, so I went hunting through the instruction manual for the company's phone number. However, on the warranty and support information page, I found the following, scrawled in a strange, dark reddish, congealed ink of some sort:

2 - 3 + 4 = ?

80 - 17 = ?
4 - 1 + 2 - _ = ?

Curious, I entered in the calucluations... I didn't know what the underscore meant, so just ignored it, but regardless, the results were even more broken than my initial test calculations. By this stage I was bored, so I shrugged it off and went to bed.

Three weeks later, being the brilliant detective I am, I suddenly realised that the hand written message carried a cleverly concealed secret message. I immediately phoned the police department and was hailed as a hero.

What was wrong with the calculator? And, what was the mysterious hidden message?

Answer

The problem with the calculator is:

It adds/subtracts the segments of the display, not the numbers.

So, 4 + 3 is indeed equal to 9

The hidden message, then, is:

HELP, all in capitals. The underscore in the last sum is the bottom line segment on the display.

Example for the first sum :
http://www.uize.com/examples/seven-segment-display.html
Since it's clearly been written in blood, something foul is afoot at the calculator factory

Sorry, don't have access to much by way of image-editing stuff atm, so can't do the pretty pictures.