Evidence changes the modal reading
A sentence about an open door can support a conclusion that someone is present. A posted rule can instead make the same modal express a requirement. Do not assign fixed percentages to modal verbs: evidence, context and negation affect the strength of the claim.
Die Tür ist offen. Jemand muss hier sein.
The door is open. Someone must be here.
A past inference and past necessity have different structures
In a past inference, the main event is expressed by a perfect infinitive after the finite modal. In a modal perfect, the finite auxiliary comes first and the final modal stays an infinitive. Compare the complete verb groups before deciding what the speaker means.
Sie muss gegangen sein. Sie hat gehen müssen.
She must have left. She had to leave.
Negating necessity does not deny possibility
Muss nicht can leave alternative explanations open. Kann nicht can exclude a possibility. If evidence is limited, a cautious expression or an explicit uncertainty phrase can make the reasoning clearer than a strong assertion.
Das muss nicht falsch sein.
That need not be wrong.
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References
Original explanations and examples informed by grammar references. Level placement is an editorial teaching choice.