Yes it is a fungus but not one I recognise, sorry! It looks more like a truffle than anything but they grow under the ground and smell of onions...
For identification you need to see the underside of the fungus, ie whether it had a stalk fr example, so if you found a fungus group and wanted to ask them you would need to lever up the whole thing carefully.
You have not said on your profile whereabouts you live so I can't help by suggesting a local fungus group that could help you but your local library might be able to give you contact details of your nearest one. Don't delay though because they soon disappear again.

welcome to GoY,
Yes it is a toadstool/mushroom fruiting body as mentioned.

Maybe Scaly Earthball (Scleroderma verrucosum) - see site: https://www.first-nature.com/fungi/scleroderma-verrucosum.php

Or Common Earthball (Scleroderma citrinum) - see site:
http://www.essexviews.uk/photos/Essex%20Nature/Fungi/slides/common-earthball-split-mushroom.html

That's a good thought - or how about Psylolithus arhizuz, which is rather knobbier? Can you gently lever it up and see what sort of stem it has?

Hello, thank-you for your answers and I would imagine it is a toadstool/mushroom fruiting body, just curious as to which one.

The images I looked up for the Common Earthball or Scaly Earthball did not seem quite right.

Also the internet gave me no feedback for the name Psylolithus arhizuz, so I'm not sure what went wrong there, but I still cannot compare them.

Unfortunately as my Sister lives 200 odd miles away so I don't have the option to look closer myself and she is reluctant to disturb it, so the type of root not an option.

However, she did send me another picture when it opened and this time has added her car keys next to it for scale - I have to admit myself I did not realise it was so small, so I am hoping to be able to post that picture now, to see if that helps.

So the verdict is still out and suggestions still welcomed