Can the IMP do that?
At least for now, I think the answer is no. The Imp talks directly to the Electric Imp servers, so it isn’t open to your locally networked devices. You would have to have your local devices talk to the Imps agent or vice-verse. While this might seem like a downside, I look at it as an advantage. Electric Imp provides HTTPS communication to the Imp (and hopefully soon even more encryption services) so your Imp communications are secure.
Your question is a good one though, as we need to find good ways of having devices that are near each other, both know it, and be able to communicate.
So its not easy fro an imp to start ‘asking around’ who is up for a chat.
The main idea behind this for me would be a topline awareness system.
E.g. and IMP could sniff, see that there is a wireless fridge, oven, etc. therefore could have contextual functionality or say ‘hello kitchen’. Just to give a silly example. But basically doing basic awareness without full-on systems capability.
Are there other IoT boxes that can easily do that or would i have to run an app on a laptop in the corner?
I started doing a little bit of that sort of thing with with a reverse proxy. Mine is running on a laptop, but I’d like to get it to run on one of the Raspberry Pi’s that I have sitting around not doing much. With a reverse proxy, your Imp’s agent could talk to your local devices. Also, you can use your own domain name which is kind of fun… like toaster.joelwehr.com (not real, though some other things are 
) There is an iOS app called IoT Monitor that seems to be doing something like that, but I haven’t look closely at exactly what it does.
Yes, I am a bit surprised I can’t find more or easier code of doing exactly that, creating a list of existing IPs and MAC addresses of all devices around you. Too paranoid? Not enough IoT yet? Weird.
The IoT things I found looks more like a list of sensor feeds, not an ad-hoc scanning.
So are there no squirrel based arp scanners or so?
Things like uPnP allowed that, but pretty much every implementation was riddled with security holes, so few people think of implementing it any more. After discovery then there’s the issue of what you do with it next… local web interface? Is it supposed to be driven by a machine or a human? etc etc etc…
I see and I understand. All I needed it for was a little IoT top level scanner.