Reopening assessments for undisclosed expenditure: officer may serve notice and reassess where chargeable expenditure escaped assessment. If the assessing officer has a reason to believe that chargeable expenditure has escaped assessment, he may serve a notice containing requirements akin to the return notice and proceed to assess or reassess. If the belief stems from the assessee's omission to file a return or failure to disclose material facts, the officer may act at any time; where the belief is based on information in the officer's possession without such omission, the officer's power to issue notice and reassess is confined to a statutory time limit measured from the end of the assessment year.