It is so kind of you to leave a tip when you stay at a hotel and even more so that you want to follow up on what you consider an oversight. @JLeslie‘s suggestion of sending a letter is ideal. Address it to the General Manager. Include a few dollars, if you wish. While the money may buy them a soda or go towards bus fare or gas, the recognition is invaluable.
@rooeytoo Tipping expectations vary by country. In the US, a tip is expected in a restaurant. Servers are legally paid below the federal and state minimum wages because of this. Other than that, the US custom is to tip taxi drivers, vallet car parkers, bell staff, etc. The general practice is to tip anyone that assists you.
If anyone wants to make a boatload of money, become a hospitable bell person at a posh hotel. The amount of tips that they get, under the table, is astounding.
With hotel room attendants, a tip isn’t expected, but it sure is a nice touch. These are the people that are cleaning up after us. I suspect that the reason they don’t receive more tips is that they rarely come in contact with the guests they are servicing, so there is no personal connection.