I was contacted lately by a couple of "Expert Networks" wanting me to provide consulting services. It turns out that this is quite common and as a retired professional, an easy way to make a little extra money on the side.
Expert Networks provide consulting services to financial analysts and businesses. Usually, they want to set up a phone call between an expert and one of their clients so the client can ask questions concerning a company or industry. For example: "Will the recent spectrum auction in the USA change the capital investments of bidders and in what way?" The financial analyst might be part of a team looking at tens of millions of dollars in investment and a one hour phone call costing a few hundred dollars is a useful part of the analysis.
You sign up with the Expert Network, like Coleman Research Group, and fill out forms detailing your expertise, then they will contact you if needed. Your fee will likely be in the hundreds of dollars per hour and you will be paid as an independent contractor via the "1099" process.
Of course you should be careful about these interactions. You cannot disclose material non-public information (which you probably don't have anymore since you are retired). You cannot disclose information that is protected by a non-disclosure agreement with a previous employer. There are a few other rules also. The Expert Network company will make sure you know these restrictions and make you sign an agreement that you will not violate any laws.
I was able to make a few hundred dollars from an interaction with one of these companies and it may be something that retired folks should consider as a way to keep your hand in the game and make a few extra bucks.