This is a very interesting ethical discussion - bar the uninteresting mudslinging!
Well, let's say that the OP really needed what she found; that it would mean a big difference to her and her family. Certainly her desire to keep it would imply that she will indeed find it of great benefit.
Let's say for instance that she and her family hadn't eaten properly for three days, just a loaf of bread and half a tin of beans for example - let's say they had an unexpected bill earlier on in the week - and they found a voucher for 50 quid for tescos?
Is she being amoral or unethical to want to keep that and not to hand that in?
How about they found a bag of crisps?
A woolly hat?
A lollipop?
A pair of gloves?
A newspaper?
A book?
Where, if anywhere do we draw the line - are we condemning her, as 'amoral' and 'unethical' because she has desire to keep the object she sees would be of benefit to her - or do we only think it's wrong of her because the value of the object is 'great' in our minds - we do not know after all what the gizmo is.
Karma works many ways - to find an object is as karmically linked as to lose an object.
Neither have to be negative or positive - both loss and gain can be neutral.
OP you have to decide if the benefit is greater to you to keep the object, knowing that someone who might have noticed it missing is going without it, or to leave the object in a place where, if the person who lost it notices it's missing will know where to find it.
Personally, and speaking from my life, I would hand it in to the police, knowing that there is a procedure in operation where you may see it 'returned' to you.
Without that certainty ( and going on the advice from your RL police friend) I can certainly see why you may be reluctant to let it go into the police system.
Some posters here have posted with (self claimed) expert advice on the workings of the lost and found system, and if you are in any doubt of whether you can trust your police, I suppose you can always ask at your local station what their policy is, before you commit to the proceedure.
I do hope that things improve for you and your family. I know it's a difficult time for a lot of people, and you must be in a difficult position if this find means so much to you.
I would be loath to pronounce the OP as amoral or unethical if she chooses to keep the gizmo - these alienating black and white judgments are not useful terms in a discussion about morals or ethics, and they don't help our OP!
And these harsh judgment words such as these labels, are karmically very bad for everyone, especially those who say them!