Maybe. Maybe not.
Here's the law for Toronto. (I assume this to be the general location of concern.)
Trespass to Property Act, R.S.O. 1990, c. T.21 | Ontario.ca
Assuming the lot is not a garden, agricultural lot (i.e., farm), or fenced or walled-off, there may be a presumption that entry is not automatically prohibited by law.
For a vacant, un-fenced, proposed future Wal-Mart construction site, much may depend on how effectively the property is placarded (i.e., how many, and what kinds of signs, and do they meet the legal requirements).
I would not condone intentional trespass.
However, as a practical matter, an offense occurs only if someone complains or their rights are abridged in some meaningful way.
If the property owner is required to take affirmative steps to curb trespass, and they fail to do so, it's a little difficult to argue that they have not waived those rights.
I would also argue, as a practical matter, that Wal-Mart would likely rather avoid any negative publicity.
Now, if this is a active construction site - different story. Wal-Mart and its contractors would not have safety reasons to deny access, etc...
Ditto if Wal-Mart knows (for example) that it is building over-top of sensitive, yet undisclosed, environmental, historical, archaeological, or other "protected" environments.
In the latter case, one might even argue that the "trespassor" has a DUTY to reveal the fact that Wal-Mart is about to buldoze over say..., the last of a species! (or an indian burial ground).
Things are often not black-&-white. (Though from the original post, this is clearly about a guy who just wants to metal-detect another's property without asking for permission first.)