(Dow Jones) When someone dies, heirs can be surprised to learn what the estate holds for them. And they aren't alone: Estates are often shrouded in some mystery even for the people who plan and manage them.

It is logical that an estate plan should offer a clear map of what a person owns, but this is not always the case. Sometimes that person doesn't have an accurate balance sheet to start with, and chooses not to update it or to share every detail. Bad communication between attorneys and advisors may also create trouble.

In one case, says Don R. Weigandt, a wealth advisor in the Los Angeles office of J.P. Morgan Private Bank, the children of a man who was supposedly quite wealthy discovered at his death that the family business was no longer worth much and most liquid assets had been used up. The children knew he had stashed away some valuable jewels in his safe deposit box, and in fact had actually seen them there. But, when it was opened, only some old papers remained.

"We never did find out what happened to them," says Weigandt.

A lawyer who drafts an estate plan may try for a full accounting of assets and fail to get it through no fault of his own. Making a plan then can be like moving around boxes, the exact contents of which aren't known.

"Life is full of surprises," says Carol Kroch, managing director, charitable trusts and head of wealth and financial planning at Wilmington Trust. "Nobody walks in and says to their lawyer, 'I'm not telling you about the other $50,000.'"

A car collection, art work or even a house can go unmentioned; there are many things that can fall between the cracks. People may withhold information because they do not entirely trust an advisor, or because they are embarrassed to talk about money.

A good estate planner, says Duncan E. Osborne, a partner at Osborne, Helman, Knebel & Deleery, chooses not to represent someone he feels is not forthcoming. Still, ambiguities about net worth are often at the heart of estate planning.

A big part of the job is to value assets properly, and that task is an art, not a science. A person may not even know himself what he is worth if, for example, he owns a lot of different partnership interests.

M. Holly Isdale, a consultant to high-net-worth people, has set up "doomsday" books for clients that compile information for estate planning but also can be helpful for keeping tracks of assets in portfolios.