Is the stranger ascending the stairs really lost/curious/bored or is it a rival hoping to get a peek into your organisation? Protect against every eventuality with a couple of straightforward steps: Get a guest book; Invest in some named visitor badges; Ensure that reception is informed of all meetings. It may sound silly - a hard-nosed snoop won't find it at all difficult to get around these precautions - but you may just deter the more opportunistic spies.
You would be amazed how many companies fail to close email accounts or change passwords for sensitive data once an employee moves on. It's not that difficult to put a redirect on a defunct email account and change the login information. We're sure none of your staff would ever bear a grudge and try and use this data for ill - but prevention is better than cure.
UK-based gadget firm Spymaster has more cool contraptions than Q. If you want to go 'next level' in your espionage prevention tactics, invest in a pinhole camera - or any number of camouflaged recording devices.
There's nothing like an iron-clad non-disclosure agreement (NDA) to help you sleep well at night. Got a top secret invention in the pipeline that would make Willy Wonka weep with envy? Make sure you slap NDAs on everyone involved.
You can't be spied on if there's nothing to steal. Embrace the new age for open source and the democratic 'my data is your data' approach and take a collaborative stance rather than erecting a walled garden. Just sayin'.