Marking in the house

Question
My 2 year old black lab is house trained and recently starting "marking" in the house when we leave.  Any suggestions on why he is doing this?  We have pointed it out to him and told him bad..he seems to understand because his ears drop.  

Answer
Don't count on it.  His ears drop because you are unhappy with him and he may not know why.

Having a good pack structure reduces such problems.  The dogs see all the
people and dogs in the household as a pack with each having their own rank in
the pack and a top dog.  Life is much easier if the 2 legged pack members
outrank the 4 legged ones.  You can learn to play the role of top dog by
reading some books or going to a good obedience class. A good obedience class
or book is about you being top dog, not about rewarding standard commands with
a treat. Start at http://www.dogsbestfriend.com/

Much of it could be about him maturing and wanting to claim a higher ranking in the pack.  If not already, neuter him.  If you were thinking of breeding him, he has just proven himself unsuitable.  Here are some other ways to show you are top dog, http://www.dogbreedinfo.com./topdogrules.htm

While waiting for all the above to have much effect, you must constantly watch him or restrict him.  Use gates or closed doors to keep him in the same room as you.  I have a short chain fastened to our one computer desk.  Other dogs may
not be as bad as the young Labs I am plagued with.  Still your house and dog
will be much safer with the dog in a crate when you are away.  The dog may be
happier in its den than loose in the house.  It relaxes, it feels safe in its
den.  It rests, the body slows down reducing the need for water and relieving
its self.  Dogs that have been crated all along do very well.  Many of them
will rest in their crates even when the door is open.  I think the plastic
ones give the dog more of a safe, enclosed den feeling.  They are harder for
dogs to open too.    Metal ones can be put in a corner or covered with
something the dog can't pull in and chew.  Select
a crate just big enough for the full grown dog to stretch out in.

Leave it some toys.  Perhaps a Kong filled with peanut butter.  Don't leave
anything in the crate the dog might chew up.  It will do fine without even any
bedding.  You will come home to a safe dog and a house you can enjoy.

A dog that has not been crated since it was little, make take some work.
Start out just putting its toys and treats in the crate.  Praise it for going
in.  If you have been able to trust it with any bedding, put that in the crate.
Feed it in the crate.  This is also an easy way to maintain order at feeding
time for more than one dog.

Most of the dog training world is moving away from harsh corrections.  I still think there is a case for them with a dog that marks.  Catching him in the act of marking calls for stronger corrections than the otherwise effective "Bad dog!".  When it misbehaves, throw it on its back, and growl "Bad dog!" right in its face.  Hold it down until it lifts one back leg to show submission.  Others like the squirt bottle.  Fill it with water and a little vinegar or lemon juice.  Give it a squirt in the face as soon as it misbehaves.  Dogs hate that.