I agree with Margaret that the lack of a lockbox doesn't mean anything.
However you must be given 24 hour notice before a potential buyer can look at your unit. 9 South doesn't show up in the MLS at all, so I don't know how many units are in your building... the fewer there are, the more likely a potential buyer will want to see yours on a first showing. Unfortunately, it is entirely possible that a landlord would put a property on the market without telling the tenants until the first buyer showing is scheduled... and that could be when you first find out. (It happened to a friend of mine.)
Is there a reason why you're worrying about this? Has the landlord been having difficulties paying for heat or other bills? Have "insurance companies" been around, taking pictures? Often that's an indication that something is up... the so-called insurance company is actually an appraiser or Realtor.
If the building does go on the market to sell, your best bet is to make sure you are current with your rent, and that you're keeping your apartment in excellent shape. A new landlord won't usually want to lose a good tenant.
If there's a possible foreclosure in the horizon, you'll still want to stay current on your rent in order to have good references when it's time to look for a new place to live.
My husband John has a property management company (http://www.onthespotrentals.com), with a wonderful 2 bedroom, 2 story duplex apartment in Freeport about to come on the market... we just saw it last night with the landlord. He also has other available apartments all over Androscoggin, Cumberland, Kennebec & York Counties.
If you find yourself in a difficult position, please give him a call at 207.713.0674.
In the meantime, unless there are some things going on that you haven't mentioned, it sounds like you're okay - no sense in worrying about something that may not be an issue at all.
Take good care of yourself.
Linda Snyder, Associate Broker
Allen & Selig Realty
One Walnut Hill Commons
N Yarmouth 04097